The present invention relates to a photomask, either binary or phase shift, which includes a hard mask layer, the use of which improves the uniformity of critical dimensions on the photomask.
Photomasks are used in the semiconductor industry to transfer microscale images defining a semiconductor circuit onto a silicon or gallium arsenide substrate or wafer. A typical binary photomask is comprised of a transparent quartz substrate and chrome (Cr) opaque material that includes an integral layer of chrome oxide (CrO) anti-reflective (AR) material. The pattern of the Cr opaque material and CrO AR material on the quartz substrate is a scaled negative of the image desired to be formed on the semiconductor wafer.
As shown in FIG. 1, a prior art blank photomask 20 is comprised of four layers. The first layer 2 is a layer of quartz, commonly referred to as the substrate, and is typically approximately one quarter inch thick. Affixed to the quartz substrate 2 is a layer of Cr opaque material 4 which typically is approximately 900 xc3x85 to 1000 xc3x85 thick. An integral layer of CrO AR material 6 is formed on top of the layer of Cr opaque material 4. The layer of CrO AR material is typically approximately 100 xc3x85 thick. A layer of photosensitive resist material 8 resides on top of the CrO AR material 6. The photosensitive resist material 8 is typically a hydrocarbon polymer, the various compositions and thicknesses of which are well known in the art.
The desired pattern of Cr opaque material to be created on the photomask may be defined by an electronic data file loaded into an exposure system which typically scans an electron beam (E-beam) or laser beam in a raster fashion across the blank photomask. One such example of a raster scan exposure system is described in U.S. Pat. No. 3,900,737 to Collier. As the E-beam or laser beam is scanned across the blank photomask, the exposure system directs the E-beam or laser beam at addressable locations on the photomask as defined by the electronic data file. The areas of the photosensitive resist material that are exposed to the E-beam or laser beam become soluble while the unexposed portions remain insoluble. As shown in FIG. 2, after the exposure system has scanned the desired image onto the photosensitive resist material, the soluble photosensitive resist is removed by means well known in the art, and the unexposed, insoluble photosensitive resist material 10 remains adhered to the CrO AR material 6.
As illustrated in FIG. 3, the exposed CrO AR material and the underlying Cr opaque material no longer covered by the photosensitive resist material is removed by a well known etching process, and only the portions of CrO AR material 12 and Cr opaque material 14 residing beneath the remaining photosensitive resist material 10 remain affixed to quartz substrate 2. This initial or base etching may be accomplished by either a wet-etching or dry-etching process both of which are well known in the art. In general, wet-etching process uses a liquid acid solution to erode away the exposed CrO AR material and Cr opaque material. A dry-etching process, also referred to as plasma etching, utilizes electrified gases, typically a mixture of chlorine and oxygen, to remove the exposed chrome oxide AR material and chrome opaque material.
A dry-etching process is conducted in vacuum chamber in which gases, typically chlorine and oxygen are injected. An electrical field is created between an anode and a cathode in the vacuum chamber thereby forming a reactive gas plasma. Positive ions of the reactive gas plasma are accelerated toward the photomask which is oriented such that the surface area of the quartz substrate is perpendicular to the electrical field. The directional ion bombardment enhances the etch rate of the Cr opaque material and CrO AR material in the vertical direction but not in the horizontal direction (i.e., the etching is anisotropic or directional).
The reaction between the reactive gas plasma and the Cr opaque material and CrO AR material is a two step process. First, a reaction between the chlorine gas and exposed CrO AR material and Cr opaque material forms chrome radical species. The oxygen then reacts with the chrome radical species to create a volatile which can xe2x80x9cboil offxe2x80x9d thereby removing the exposed CrO AR material and the exposed Cr opaque material.
As shown in FIG. 4, after the etching process is completed the photosensitive resist material is stripped away by a process well known in the art. The dimensions of the Cr opaque material on the finished photomask are then measured to determine whether or not critical dimensions are within specified tolerances. Critical dimensions may be measured at a number of locations on the finished photomask, summed, and then divided by the number of measurements to obtain a numerical average of the critical dimensions. This obtained average is then compared to a specified target number (i.e., a mean to target comparison) to ensure compliance with predefined critical dimensions specifications. Additionally, it is desired that there is a small variance among the critical dimensions on the substrate. Accordingly, the measured critical dimensions typically must also conform to a specified uniformity requirement. Uniformity is typically defined as a range (maximum minus minimum) or a standard deviation of a population of measurements.
Another type of known photomask used for transferring images to a semiconductor wafer is commonly referred to as a phase shift photomask. Phase shift photomasks are generally preferred over binary photomasks where the design to be transferred to the semiconductor wafer includes smaller, packed together feature sizes which are below the resolution requirements of optical equipment being used. Phase shift photomasks are engineered to be 180 degrees out of phase with light transmitted through etched areas on the photomask so that the light transmitted through the openings in the photomask is equal in amplitude.
One type of known phase shift photomask is commonly referred to as an embedded attenuated phase shift mask (xe2x80x9cEAPSMxe2x80x9d). As shown in FIG. 10, a typical blank EAPSM 31 is comprised of four layers. The first layer is a typically a substantially transparent material 33 (such as quartz, for example) and is commonly referred to as a substrate. The next layer is typically an embedded phase shifting material (xe2x80x9cPSM layerxe2x80x9d) 35, such as molybdenum silicide (MoSi), tantalum silicon nitride (TaSiN), titanium silicon nitride (TiSiN) or zirconium silicon oxide (ZrSiO) and other known phase materials. The next layer is typically an opaque material 37, such as chromium, which may optionally include an anti-reflective coating such as chromium oxynitride (CrON). The top layer is a photosensitive resist material 39.
The method for processing a conventional EAPSM is now described. As with binary photomasks, the desired pattern of opaque material 37 to be created on the EAPSM 31 is scanned by an electron beam (E-beam) or laser beam in a raster or vector fashion across the blank EAPSM 31. As the E-beam or laser beam is scanned across the blank EAPSM 31, the exposure system directs the E-beam or laser beam at addressable locations on the EAPSM 31. The areas of the photosensitive resist material 39 that are exposed to the E-beam or laser beam become soluble while the unexposed portions remain insoluble.
As is done with binary photomasks and as shown in FIG. 11, after the exposure system has scanned the desired image onto the photosensitive resist material 39, the soluble photosensitive resist material is removed by means well known in the art, and the unexposed, insoluble photosensitive resist material 39xe2x80x2 remains adhered to the opaque material 37. Thus, the pattern to be formed on the EAPSM 31 is formed by the remaining photosensitive resist material 39xe2x80x2.
The pattern is then transferred from the remaining photoresist material 39xe2x80x2 to the opaque layer 37 and PSM layer 35 via the known etching techniques described above, with portions of the opaque layer 37 and PSM layer 35 not covered by the remaining photoresist 39xe2x80x2 being etched away. After etching is completed, the remaining photoresist material 39xe2x80x2 is stripped or removed and the EAPSM 31 is finished or completed, as shown in FIG. 12. In the finished EAPSM 31xe2x80x2, the pattern as previously reflected by the PSM 35 and opaque 37 materials is located in regions where the remaining photoresist 39xe2x80x2 remain after the soluble materials were removed in prior steps.
To create an image on a semiconductor wafer, the photomask (e.g., binary or phase shift) is interposed between the semiconductor wafer, which includes a layer of photosensitive material, and an energy source commonly referred to as a Stepper, as shown in FIG. 13. The energy generated by the Stepper passes through the transparent portions of the substantially transparent substrate not covered by the opaque material (and, if utilized, the anti-reflective and/or phase shift material) and causes a reaction in the photosensitive material on the semiconductor wafer. It is noted that anti-reflective material is useful in preventing most, but not all, of the incident energy from being reflected back into the Stepper. If excess energy is reflected back into the Stepper a degraded image will be created in the photosensitive resist material on the semiconductor wafer surface, thereby resulting in a degradation of performance of the semiconductor device. Energy from the Stepper is inhibited from passing through the areas of the photomask in which the opaque material.
The etch rate of the plasma etching process described above (and hence the uniformity of the critical dimensions) is dependent on the desired pattern to be formed in the opaque material (anti-reflective material, if used) and phase shift material (in the case of an EAPSM). In areas of the photomask (either binary or phase shift) where a substantial portion of opaque material and phase shift material (and anti-reflective materials, if used) are to be removed (i.e., macro loading), the etching process may take longer than in areas of the photomask in which small portions of opaque material and phase shift material (and anti-reflective material, if used) are to be removed. Likewise, there may be differences in etch rate for micro loading conditions in which the etch rate is different between isolated and dense features in the same general area. These differing etch rates make it more difficult for the finished photomask to conform to a specified uniformity requirement. Additionally, the above described etching process can also cause variances in critical dimensions because the photosensitive resist material is not entirely impervious to the plasma gases.
While the prior art is of interest, the known methods and apparatus of the prior art present several limitations which the present invention seeks to overcome.
Accordingly, it is an object of the present invention to provide a blank phase shift photomask which includes a layer of hard mask material thereby enabling the critical dimensions of a finished photomask to be more uniform.
It is a further object of the invention to provide a method for manufacturing a finished phase shift photomask having improved uniformity of critical dimensions.
It is still further an object of the present invention to provide a finished phase shift photomask having improved uniformity in critical dimensions and improved anti-reflection properties thereby reducing the amount of error introduced by the basic lithography process.
It is another object of the present invention to solve the shortcomings of the prior art.
Other objects and advantages of the present invention will become apparent from the foregoing description.
It has now been found that the above and related objects of the present invention are obtained in the form of several related aspects, including an improved blank EAPSM, a method for forming an image on the blank EAPSM, a finished EAPSM, and a method of making a semiconductor or integrated circuit using the finished EAPSM.
The blank EAPSM comprises a photosensitive resist material layer, a hard mask layer underlying the photosensitive resist material layer, an opaque layer underlying the hard mask layer, a phase shift layer underlying the opaque layer and a substantially transparent substrate layer underlying the phase shift layer. The hard mask layer is made from materials which are selectively resistant to etching in the blank photomask. The opaque layer may optionally include an anti-reflective layer if needed or desired.
The method for creating an image on the EAPSM of the present invention comprises the steps of: creating a patterned image in the photosensitive resist layer; removing portions of the photosensitive resist layer that do not correspond to the patterned image thereby exposing portions of the hard mask layer not corresponding to the patterned image; removing the exposed portions of the hard mask layer that do not correspond to the patterned image thereby exposing portions of the opaque layer not corresponding to the patterned image; removing the exposed portions of the opaque and the phase shift layer underlying the hard mask portions that do not correspond to the patterned image thereby exposing portions of the substantially transparent layer that do not correspond to the patterned image; and removing the photosensitive resist layer. Where the EAPSM of the present invention includes an anti-reflective layer, the method of the present invention further comprises the step of removing the anti-reflective layer after the hard mask layer is removed (either prior to removing the opaque layer or together with the opaque layer).
The finished EAPSM made by the above method comprises a substantially transparent substrate; a patterned layer of phase shift material affixed to the substrate; a patterned layer of opaque material (and anti-reflective material, if used or needed) affixed to the patterned layer of phase shift material; and a patterned layer of hard mask material affixed to the layer of the opaque material (or anti-reflective layer, if used), wherein the pattern formed in the phase shift material, opaque material and the hard mask material corresponds to a scaled negative or positive of the image to be formed on the image plane.
The method for manufacturing a semiconductor using the finished EAPSM of the present invention comprises the steps of: interposing a finished EAPSM between a semiconductor wafer and an energy source; generating energy in the energy source; transmitting the generated energy through a pattern formed in the EAPSM; and etching an image on the semiconductor wafer corresponding to the pattern formed in the opaque and the hard mask layers of the finished photomask. The finished EAPSM comprises a substantially transparent substrate, a patterned layer of phase shift material affixed to the substantially transparent substrate, a patterned layer of opaque material affixed to the phase shift material, and a patterned layer of hard mask material affixed to the layer of opaque material. The pattern formed in the phase shift layer, opaque layer and hard mask layer corresponds to either a scaled negative or positive of the image to be formed on the image plane.